Video: On guard for election

Many U.S. intelligence analysts believe terrorists tried to influence Spain's elections by attacking in March and that a terror group tried to do the same in Russia by crashing two planes in August.

That's one reason why U.S. security officials worry al-Qaida could be planning an attack to disrupt the U.S. elections — even though the flood of intelligence contains nothing specific.

"They've not received any additional, specific information beyond what they had in the spring time frame," says Roger Cressey, NBC News Terrorism Analyst. "There's still this general concern about an attack in the fall that could be timed toward the election."

Even so, officials at the Homeland Security Department believe they have no choice but to act as though an attack is in the works. The government plans an unprecedented deployment of federal agents, thousands nationwide — a highly visible show of determination.

Among the tactics:

Agents will review thefts of rental trucks and limousines and even unconfirmed reports of stolen delivery service uniforms.

"So a substantial amount is being done, both from a deterrent-preventive method and also from a disruptive method," says Under Secretary for Border and Transportation Security Asa Hutchinson.

And while no intelligence suggests an attack is more likely on Election Day, some states, like Ohio and Minnesota, are urging poll workers to pay closer attention to potential trouble.

With more than 180,000 polling places nationwide, voting day security will largely be a local problem.

And sensitive to concerns that this October push could bring criticism — of the administration trying to scare the electorate into voting Republican — security officials are going out of their way to insist that they've been planning this final push since the spring.